Depressed center grinding wheel

ABSTRACT

A depressed center grinding wheel comprises an abrasive disc. The abrasive disc comprises a working layer, an intermediate layer, a back layer, and at least two reinforcing scrims. The working layer comprises first abrasive particles retained in a first binder material. The first abrasive particles include from 40 to 100 weight percent of first shaped abrasive particles. The back layer comprises second abrasive particles retained in a second binder material. The second abrasive particles include first crushed abrasive particles and are essentially free of shaped abrasive particles. The intermediate layer is disposed between the working layer and the back layer. The intermediate layer comprises third abrasive particles retained in a third binder material. The third abrasive particles include 25 to 75 weight percent of second shaped abrasive particles.

TECHNICAL FIELD

The present disclosure relates to depressed center grinding wheels.

BACKGROUND

Depressed center grinding wheels are often used in combination with ahandheld portable grinder held by an operator, either at an angle of 90degrees (e.g., when used as a cut of wheel) or at an angle of up toabout 30 degrees (e.g., when used for grinding welding beads, flash,gate, and risers off of castings), more typically about 15 degrees,relative to the surface of the workpiece being abraded. Depressed centerwheels may also be referred to in the abrasive art as raised hub wheelsor by their shape designation of “Type” (e.g., Types 27, 28, and 29),with Type 27 being the most popular.

The depressed center design allows a flange/lock nut to recess withinthe wheel so that it can be used for various grinding and cuttingapplications. Bonded abrasive articles have abrasive particles bondedtogether by a bonding medium. The bonding medium is typically an organicresin, but may also be an inorganic material such as a ceramic or glass(i.e., vitreous bonds). Examples of bonded abrasive articles includestones, hones, and abrasive wheels such as, for example, grinding wheelsand cut-off wheels.

Grinding wheels are of various shapes may be, for example, driven by astationary-mounted motor such as, for example, a bench grinder, orattached and driven by a hand-operated portable grinder. Hand-operatedportable grinders are typically held at a slight angle relative to theworkpiece surface, and may be used to grind, for example, welding beads,flash, gates, and risers off castings.

SUMMARY

In recent years there have efforts to include shaped abrasive particlesin various grinding wheels (e.g., depressed center grinding wheels);however, the relatively high cost of such abrasive particles remains anobstacle to their widespread acceptance in the industry. It would bedesirable to have alternative constructions that can reduce cost ofgrinding wheels by reducing the amount of shaped abrasive particleswhile achieving comparable abrading performance.

Advantageously, the present disclosure solves this technical problem inthe case of depressed center grinding wheels by providing a depressedcenter grinding wheel comprising an abrasive disc having a workingsurface and a back surface opposite the working surface, wherein theworking surface has a depressed center portion, and wherein the abrasivedisc comprises:

a working layer comprising first abrasive particles retained in a firstbinder material, the first abrasive particles comprising first shapedabrasive particles, wherein the first shaped abrasive particles comprisefrom 40 to 100 weight percent of the first abrasive particles;

a back layer comprising second abrasive particles retained in a secondbinder material comprising first crushed abrasive particles andessentially free of shaped abrasive particles;

an intermediate layer disposed between the working layer and the backlayer, the intermediate layer comprising third abrasive particlesretained in a third binder material, the intermediate layer comprisingsecond shaped abrasive particles and second crushed abrasive particles,wherein the second shaped abrasive particles comprise 25 to 75 weightpercent of the second abrasive particles;

a first reinforcing scrim sandwiched between the back layer and theintermediate layer; and

a second reinforcing scrim adjacent one of:

-   -   the back layer opposite the intermediate layer;    -   the intermediate layer opposite the back layer; or    -   the working layer opposite the intermediate layer.

Depressed center grinding wheels according to the present disclosure areuseful; for example, for abrading a surface of a workpiece.

Accordingly, in another aspect, the present disclosure provides a methodof abrading a workpiece, the method comprising contacting a workpiecewith the working surface of a depressed center grinding wheel accordingto the present disclosure and moving the working surface relative to theworkpiece to abrade the workpiece.

As used herein, the term “nominal” means: of, being, or relating to adesignated or theoretical size and/or shape that may vary somewhat fromthe actual (e.g., within a manufacturing process tolerance). As usedherein, the term “shaped abrasive particle” refers to an abrasiveparticle (e.g., a ceramic abrasive particle) with at least a portion ofthe abrasive particle having a nominal predetermined shape correspondingto a mold cavity used to form a precursor shaped abrasive particle,which is then calcined and sintered to form the shaped abrasiveparticle. Shaped abrasive particle as used herein excludes abrasiveparticles shaped solely by a mechanical crushing process.

As used herein, the term “crushed abrasive particle” refers to anabrasive particle shaped solely by a mechanical crushing process.

As used herein, the term “essentially free of” means containing lessthan 5 weight percent of (preferably less than 1 weight percent of, oreven free of).

Features and advantages of the present disclosure will be furtherunderstood upon consideration of the detailed description as well as theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an exemplary depressed centergrinding wheel 100 according to the present disclosure.

FIGS. 2A-2F are schematic cross-sectional representations showingvarious exemplary configurations of scrim placement in exemplarydepressed center grinding wheels 100 a-100 f.

FIG. 3 is a schematic perspective view of exemplary shaped abrasiveparticle 300.

FIG. 4 is a schematic side view showing a depressed center grindingwheel 100 abrading a workpiece 400 according to the present disclosure.

Repeated use of reference characters in the specification and drawingsis intended to represent the same or analogous features or elements ofthe disclosure. It should be understood that numerous othermodifications and embodiments can be devised by those skilled in theart, which fall within the scope and spirit of the principles of thedisclosure. The figures may not be drawn to scale.

DETAILED DESCRIPTION

Referring now to FIG. 1, depressed center grinding wheel 100 comprisesan abrasive disc 110 having a working surface 122 and a back surface 142opposite the working surface 122. Working surface 112 has a depressedcenter portion 114. Abrasive disc 110 comprises working layer 120,intermediate layer 130, and back layer 140.

Working layer 120 comprises first abrasive particles 124 retained in afirst binder material 126. The first abrasive particles 124 comprisefirst shaped abrasive particles 125. The first shaped abrasive particles125 comprise from 40 to 100 weight percent of the first abrasiveparticles 124. The first abrasive particles 124 may also comprisecrushed abrasive particles, if desired, in amounts of up to about 60percent by weight (e.g., 5 to 60 percent by weight, 20 to 60 percent byweight, or 40 to 60 percent by weight), based on the total weight of thefirst abrasive particles. The first shaped abrasive particles 125 mayall be of the same size and shape or they may be a mixture of variousshaped abrasive particles with different sizes, shapes, and/orcompositions. In some preferred embodiments, the first abrasiveparticles are all shaped abrasive particles having the same nominal sizeand shape. The first shaped abrasive particles 125 may all be of thesame size and shape or they may be a mixture of various shaped abrasiveparticles with different sizes and/or shapes. Likewise, any optionalcrushed abrasive particles included in the first abrasive particles mayhave any size distribution and/or compositional distribution.

Back layer 140 comprises second abrasive particles 144 retained in asecond binder material 146 comprises first crushed abrasive particles148, and is essentially free of shaped abrasive particles (e.g., firstshaped abrasive particles, second shaped abrasive particles, or othershaped abrasive particles).

The second abrasive particles 144 comprise at least 95 percent by weightof first crushed abrasive particles, preferably at least 99 percent byweight, and more preferably about 100 percent by weight of first crushedabrasive particles, based on the total weight of second abrasiveparticles. Accordingly, the back layer is essentially free of shapedabrasive particles. The second abrasive particles 144 may be of anysize. The second abrasive particles 144 may have any size distributionand/or compositional distribution.

Intermediate layer 130 is disposed between the working layer 120 and theback layer 140. The intermediate layer 130 comprises third abrasiveparticles 134 retained in a third binder material 136. The intermediatelayer 130 comprises second shaped abrasive particles 135 and secondcrushed abrasive particles 138. The second shaped abrasive particles 135comprise 25 to 75 weight percent (e.g., 30 to 60 weight percent, or 40to 60 percent) of the second abrasive particles. The second shapedabrasive particles 135 may all be of the same size and shape or they maybe a mixture of various shaped abrasive particles with different sizes,shapes, and/or compositions. In some preferred embodiments, the secondabrasive particles include second shaped abrasive particles having thesame nominal size and shape. The second shaped abrasive particles 135may all be of the same size and shape or they may be a mixture ofvarious shaped abrasive particles with different sizes and/or shapes.Likewise, the second crushed abrasive particles may have any sizedistribution and/or compositional distribution.

Preferably, the first and second shaped abrasive particles are the same(i.e., same compositional, shape, and size distribution); however, theymay be different in other embodiments, if desired. Likewise, the firstand second crushed abrasive particles are preferably the same (i.e.,same compositional and size distribution); however, they may bedifferent in other embodiments, if desired.

While shaped abrasive particles in FIG. 1 are shown as verticallyaligned triangles, this is for illustration purposes only, and theshaped abrasive particles may have any orientation (e.g., randomlyaligned or aligned parallel to the backing).

Referring again to FIG. 1, depressed center grinding wheels include atleast two reinforcing scrims deployed at various locations throughoutthe grinding wheel. First reinforcing scrim 150 is sandwiched betweenback layer 140 and intermediate layer 130. Second reinforcing scrim 152(not shown) is positioned adjacent to one of: a) back layer 140 oppositeintermediate layer 130; b) intermediate layer 130 opposite back layer140; or c) working layer 120 opposite intermediate layer 130.

Optional centrally disposed arbor hole 170 extends through abrasive disc110. Optional attachment member 175 is centrally disposed, andoptionally secured by nut 180, to back surface 142 of abrasive disc 110,although this is not a requirement.

In some embodiments, the second reinforcing scrim 152 is sandwichedbetween the working layer 120 and the intermediate layer 130. Examplesinclude the embodiments shown in FIG. 2B (shown as 152 b), FIG. 2D(shown as 152 d), and FIG. 2F (shown as 152 f). In some of theseembodiments, a third reinforcing scrim 156 is bonded to the workinglayer 110 opposite the intermediate layer (e.g., see 156 b in FIG. 2B).

In some embodiments, second reinforcing scrim 152 is secured to the backlayer opposite the intermediate layer. Examples are shown in FIG. 2A(shown as 152 a), FIG. 2C (shown as 152 c), and FIG. 2E (shown as 152e). In some of these embodiments, third reinforcing scrim 156 issandwiched between the intermediate layer and the working layer (e.g.,shown as 156 e in FIG. 2E). In some of these embodiments, an optionalfourth reinforcing scrim 169 is bonded to working layer 120 oppositeintermediate layer 130 (e.g., shown as 169 e in FIG. 2E).

In FIG. 2C, optional third scrim 156 c is secured to working layer 120opposite the intermediate layer 130. Likewise, in FIG. 2F, optionalthird scrim 156 f is secured to working layer 120 opposite theintermediate layer 130.

Depressed center grinding wheels according to the present disclosure aregenerally made by compression molding, injection molding, transfermolding, or the like. The molding can be done either by hot or coldpressing or any suitable manner known to those skilled in the art.During the manufacturing, the individual components (e.g., workinglayer, intermediate layer, back layer, and scrims) are typically layeredup into a green body that is then subjected to curing conditions. Thegreen body typically contains one or more binder material precursors,either liquid organic, powdered inorganic, powdered organic, or acombination of thereof, mixed with abrasive particles (i.e., shapedabrasive particles and crushed abrasive particles selected andpositioned as described herein), and reinforcing scrims (positioned atdesired locations in the wheel). In some instances, a liquid medium(either resin or a solvent) is first applied to the abrasive particlesto wet their outer surface, and then the wetted particles are mixed witha powdered medium.

The various binder materials in the working layer, intermediate layer,and back layer (which may be the same or different, preferably the same)typically comprise a glassy inorganic material (e.g., as in the case ofvitrified abrasive wheels), metal, or an organic resin (e.g., as in thecase of resin-depressed center grinding wheels).

Glassy vitreous binders may be made from a mixture of different metaloxides. Examples of these metal oxide vitreous binders include silica,alumina, calcia, iron oxide, titania, magnesia, sodium oxide, potassiumoxide, lithium oxide, manganese oxide, boron oxide, phosphorous oxide,and the like. Specific examples of vitreous binders based upon weightinclude, for example, 47.61 percent SiO₂, 16.65 percent Al₂O₃, 0.38percent Fe₂O₃, 0.35 percent TiO₂, 1.58 percent CaO, 0.10 percent MgO,9.63 percent Na₂O, 2.86 percent K₂O, 1.77 percent Li₂O, 19.03 percentB₂O₃, 0.02 percent MnO₂, and 0.22 percent P₂O₅; and 63 percent SiO₂, 12percent Al₂O₃, 1.2 percent CaO, 6.3 percent Na₂O, 7.5 percent K₂O, and10 percent B₂O₃. During manufacture of a vitreous bonded depressedcenter grinding wheel, vitreous binder in powder form, may be mixed witha temporary binder, typically an organic temporary binder. The vitrifiedbinders may also be formed from a frit, for example anywhere from aboutone to 100 percent frit, but generally 20 to 100 percent frit. Someexamples of common materials used in frit binders include feldspar,borax, quartz, soda ash, zinc oxide, whiting, antimony trioxide,titanium dioxide, sodium silicofluoride, flint, cryolite, boric acid,and combinations thereof. These materials are usually mixed together aspowders, fired to fuse the mixture and then the fused mixture is cooled.The cooled mixture is crushed and screened to a very fine powder to thenbe used as a frit vitreous binder precursor. The temperature at whichthe frit vitreous binder precursor is matured to form a vitreous binderis dependent upon its chemistry, but typically ranges from about 600° C.to about 1800° C., although this is not a requirement.

Examples of metal binders include tin, copper, aluminum, nickel, andcombinations thereof. Metal binder materials can be formed by sinteringmetal powders, optionally containing a temporary organic binder materialthat burns off during sintering.

Organic binder materials are typically included in an amount of from 5to 30 percent, more typically 10 to 25, and more typically 15 to 24percent by weight, based of the total weight of the depressed centergrinding wheel. Phenolic resin is the most commonly used organic bindermaterial, and may be used in both the powder form and liquid state.Although phenolic resins are widely used, it is within the scope of thisdisclosure to use other organic binder materials including, for example,epoxy resins, urea-formaldehyde resins, rubbers, shellacs, and acrylicbinders. The organic binder material may also be modified with otherbinder materials to improve or alter the properties of the bindermaterial.

Useful phenolic resins include novolac and resole phenolic resins.Novolac phenolic resins are characterized by being acid-catalyzed andhaving a ratio of formaldehyde to phenol of less than one, typicallybetween 0.5:1 and 0.8:1. Resole phenolic resins are characterized bybeing alkaline catalyzed and having a ratio of formaldehyde to phenol ofgreater than or equal to one, typically from 1:1 to 3:1. Novolac andresole phenolic resins may be chemically modified (e.g., by reactionwith epoxy compounds), or they may be unmodified. Exemplary acidiccatalysts suitable for curing phenolic resins include sulfuric,hydrochloric, phosphoric, oxalic, and p-toluenesulfonic acids. Alkalinecatalysts suitable for curing phenolic resins include sodium hydroxide,barium hydroxide, potassium hydroxide, calcium hydroxide, organicamines, or sodium carbonate.

Phenolic resins are well-known and readily available from commercialsources. Examples of commercially available novolac resins include DUREZ1364, a two-step, powdered phenolic resin (marketed by Durez Corporationof Addison, Tex. under the trade designation VARCUM (e.g., 29302), orHEXION AD5534 RESIN (marketed by Hexion Specialty Chemicals, Inc. ofLouisville, Ky.). Examples of commercially available resole phenolicresins useful in practice of the present disclosure include thosemarketed by Durez Corporation under the trade designation VARCUM (e.g.,29217, 29306, 29318, 29338, 29353); those marketed by Ashland ChemicalCo. of Bartow, Fla. under the trade designation AEROFENE (e.g., AEROFENE295); and those marketed by Kangnam Chemical Company Ltd. of Seoul,South Korea under the trade designation “PHENOLITE” (e.g., PHENOLITETD-2207).

Curing temperatures of organic binder material precursors will generallyvary with the material chosen and wheel design. Selection of suitableconditions is within the capability of one of ordinary skill in the art.Exemplary conditions for a phenolic binder may include an appliedpressure of about 20 tons per 4 inches diameter (224 kg/cm²) at roomtemperature followed by heating at temperatures up to about 185° C.(degrees Celsius) for sufficient time to cure the organic bindermaterial precursor.

In some embodiments, the depressed center grinding wheels include fromabout 10 to 60 percent by weight of abrasive particles; typically 30 to60 percent by weight, and more typically 40 to 60 percent by weight,based on the total weight of the binder material(s) and abrasiveparticles.

Shaped abrasive particles composed of crystallites of alpha alumina,magnesium alumina spinel, and a rare earth hexagonal aluminate may beprepared using sol-gel precursor alpha alumina particles according tomethods described in, for example, U.S. Pat. No. 5,213,591 (Celikkaya etal.) and U.S. Publ. Patent Appl. Nos. 2009/0165394 A1 (Culler et al.)and 2009/0169816 A1 (Erickson et al.).

In some embodiments, alpha-alumina-based shaped abrasive particles canbe made according to a multistep process. Briefly, the method comprisesthe steps of making either a seeded or non-seeded sol-gel alpha aluminaprecursor dispersion that can be converted into alpha alumina; fillingone or more mold cavities having the desired outer shape of the shapedabrasive particle with the sol-gel, drying the sol-gel to form precursorshaped abrasive particles; removing the precursor shaped abrasiveparticles from the mold cavities; calcining the precursor shapedabrasive particles to form calcined, precursor shaped abrasiveparticles, and then sintering the calcined, precursor shaped abrasiveparticles to form shaped abrasive particles. The process will now bedescribed in greater detail.

The first process step involves providing either a seeded or non-seededdispersion of an alpha alumina precursor that can be converted intoalpha alumina. The alpha alumina precursor dispersion often comprises aliquid that is a volatile component. In one embodiment, the volatilecomponent is water. The dispersion should comprise a sufficient amountof liquid for the viscosity of the dispersion to be sufficiently low toenable filling mold cavities and replicating the mold surfaces, but notso much liquid as to cause subsequent removal of the liquid from themold cavity to be prohibitively expensive. In one embodiment, the alphaalumina precursor dispersion comprises from 2 percent to 90 percent byweight of the particles that can be converted into alpha alumina, suchas particles of aluminum oxide monohydrate (boehmite), and at least 10percent by weight, or from 50 percent to 70 percent, or 50 percent to 60percent, by weight of the volatile component such as water. Conversely,the alpha alumina precursor dispersion in some embodiments contains from30 percent to 50 percent, or 40 percent to 50 percent, by weight solids.

Aluminum oxide hydrates other than boehmite can also be used. Boehmitecan be prepared by known techniques or can be obtained commercially.Examples of commercially available boehmite include products having thetrade designations “DISPERAL”, and “DISPAL”, both available from SasolNorth America, Inc. of Houston, Tex., or “HiQ-40” available from BASFCorporation of Florham Park, N.J. These aluminum oxide monohydrates arerelatively pure; that is, they include relatively little, if any,hydrate phases other than monohydrates, and have a high surface area.

The physical properties of the resulting shaped abrasive particles willgenerally depend upon the type of material used in the alpha aluminaprecursor dispersion. In one embodiment, the alpha alumina precursordispersion is in a gel state. As used herein, a “gel” is a threedimensional network of solids dispersed in a liquid.

The alpha alumina precursor dispersion may contain a modifying additiveor precursor of a modifying additive. The modifying additive canfunction to enhance some desirable property of the abrasive particles orincrease the effectiveness of the subsequent sintering step. Modifyingadditives or precursors of modifying additives can be in the form ofsoluble salts, typically water soluble salts. They typically consist ofa metal-containing compound and can be a precursor of oxide ofmagnesium, zinc, iron, silicon, cobalt, nickel, zirconium, hafnium,chromium, yttrium, praseodymium, samarium, ytterbium, neodymium,lanthanum, gadolinium, cerium, dysprosium, erbium, titanium, andmixtures thereof. The particular concentrations of these additives thatcan be present in the alpha alumina precursor dispersion can be variedbased on skill in the art.

Typically, the introduction of a modifying additive or precursor of amodifying additive will cause the alpha alumina precursor dispersion togel. The alpha alumina precursor dispersion can also be induced to gelby application of heat over a period of time. The alpha aluminaprecursor dispersion can also contain a nucleating agent (seeding) toenhance the transformation of hydrated or calcined aluminum oxide toalpha alumina. Nucleating agents suitable for this disclosure includefine particles of alpha alumina, alpha ferric oxide or its precursor,titanium oxides and titanates, chrome oxides, or any other material thatwill nucleate the transformation. The amount of nucleating agent, ifused, should be sufficient to effect the transformation of alphaalumina. Nucleating such alpha alumina precursor dispersions isdisclosed in U.S. Pat. No. 4,744,802 (Schwabel).

A peptizing agent can be added to the alpha alumina precursor dispersionto produce a more stable hydrosol or colloidal alpha alumina precursordispersion. Suitable peptizing agents are monoprotic acids or acidcompounds such as acetic acid, hydrochloric acid, formic acid, andnitric acid. Multiprotic acids can also be used but they can rapidly gelthe alpha alumina precursor dispersion, making it difficult to handle orto introduce additional components thereto. Some commercial sources ofboehmite contain an acid titer (such as absorbed formic or nitric acid)that will assist in forming a stable alpha alumina precursor dispersion.

The alpha alumina precursor dispersion can be formed by any suitablemeans, such as, for example, by simply mixing aluminum oxide monohydratewith water containing a peptizing agent or by forming an aluminum oxidemonohydrate slurry to which the peptizing agent is added.

Defoamers or other suitable chemicals can be added to reduce thetendency to form bubbles or entrain air while mixing. Additionalchemicals such as wetting agents, alcohols, or coupling agents can beadded if desired. The alpha alumina abrasive particles may containsilica and iron oxide as disclosed in U.S. Pat. No. 5,645,619 (Ericksonet al.). The alpha alumina abrasive particles may contain zirconia asdisclosed in U.S. Pat. No. 5,551,963 (Larmie). Alternatively, the alphaalumina abrasive particles can have a microstructure or additives asdisclosed in U.S. Pat. No. 6,277,161 (Castro).

The second process step involves providing a mold having at least onemold cavity, and preferably a plurality of cavities. The mold can have agenerally planar bottom surface and a plurality of mold cavities. Theplurality of cavities can be formed in a production tool. The productiontool can be a belt, a sheet, a continuous web, a coating roll such as arotogravure roll, a sleeve mounted on a coating roll, or die. In oneembodiment, the production tool comprises polymeric material. Examplesof suitable polymeric materials include thermoplastics such aspolyesters, polycarbonates, poly(ether sulfone), poly(methylmethacrylate), polyurethanes, poly(vinyl chloride), polyolefin,polystyrene, polypropylene, polyethylene or combinations thereof, orthermosetting materials. In one embodiment, the entire tooling is madefrom a polymeric or thermoplastic material. In another embodiment, thesurfaces of the tooling in contact with the sol-gel while drying, suchas the surfaces of the plurality of cavities, comprises polymeric orthermoplastic materials and other portions of the tooling can be madefrom other materials. A suitable polymeric coating may be applied to ametal tooling to change its surface tension properties by way ofexample.

A polymeric or thermoplastic tool can be replicated off a metal mastertool. The master tool will have the inverse pattern desired for theproduction tool. The master tool can be made in the same manner as theproduction tool. In one embodiment, the master tool is made out ofmetal, e.g., nickel and is diamond turned. The polymeric sheet materialcan be heated along with the master tool such that the polymericmaterial is embossed with the master tool pattern by pressing the twotogether. A polymeric or thermoplastic material can also be extruded orcast onto the master tool and then pressed. The thermoplastic materialis cooled to solidify and produce the production tool. If athermoplastic production tool is utilized, then care should be taken notto generate excessive heat that may distort the thermoplastic productiontool limiting its life. More information concerning the design andfabrication of production tooling or master tools can be found in U.S.Pat. No. 5,152,917 (Pieper et al.); U.S. Pat. No. 5,435,816 (Spurgeon etal.); U.S. Pat. No. 5,672,097 (Hoopman et al.); U.S. Pat. No. 5,946,991(Hoopman et al.); U.S. Pat. No. 5,975,987 (Hoopman et al.); and U.S.Pat. No. 6,129,540 (Hoopman et al.).

Access to cavities can be from an opening in the top surface or bottomsurface of the mold. In some instances, the cavities can extend for theentire thickness of the mold. Alternatively, the cavities can extendonly for a portion of the thickness of the mold. In one embodiment, thetop surface is substantially parallel to bottom surface of the mold withthe cavities having a substantially uniform depth. At least one side ofthe mold, that is, the side in which the cavities are formed, can remainexposed to the surrounding atmosphere during the step in which thevolatile component is removed.

The cavities have a specified three-dimensional shape to make the shapedabrasive particles. The depth dimension is equal to the perpendiculardistance from the top surface to the lowermost point on the bottomsurface. The depth of a given cavity can be uniform or can vary alongits length and/or width. The cavities of a given mold can be of the sameshape or of different shapes.

The third process step involves filling the cavities in the mold withthe alpha alumina precursor dispersion (e.g., by a conventionaltechnique). In some embodiments, a knife roll coater or vacuum slot diecoater can be used. A mold release can be used to aid in removing theparticles from the mold if desired. Typical mold release agents includeoils such as peanut oil or mineral oil, fish oil, silicones,polytetrafluoroethylene, zinc stearate, and graphite. In general, moldrelease agent such as peanut oil, in a liquid, such as water or alcohol,is applied to the surfaces of the production tooling in contact with thesol-gel such that between about 0.1 mg/in² (0.02 mg/cm²) to about 3.0mg/in² 0.46 mg/cm²), or between about 0.1 mg/in² (0.02 mg/cm²) to about5.0 mg/in² (0.78 mg/cm²) of the mold release agent is present per unitarea of the mold when a mold release is desired. In some embodiments,the top surface of the mold is coated with the alpha alumina precursordispersion. The alpha alumina precursor dispersion can be pumped ontothe top surface.

Next, a scraper or leveler bar can be used to force the alpha aluminaprecursor dispersion fully into the cavity of the mold. The remainingportion of the alpha alumina precursor dispersion that does not entercavity can be removed from top surface of the mold and recycled. In someembodiments, a small portion of the alpha alumina precursor dispersioncan remain on the top surface and in other embodiments the top surfaceis substantially free of the dispersion. The pressure applied by thescraper or leveler bar is typically less than 100 psi (0.7 MPa), lessthan 50 psi (0.3 MPa), or even less than 10 psi (69 kPa). In someembodiments, no exposed surface of the alpha alumina precursordispersion extends substantially beyond the top surface to ensureuniformity in thickness of the resulting shaped abrasive particles.

The fourth process step involves removing the volatile component to drythe dispersion. Desirably, the volatile component is removed by fastevaporation rates. In some embodiments, removal of the volatilecomponent by evaporation occurs at temperatures above the boiling pointof the volatile component. An upper limit to the drying temperatureoften depends on the material the mold is made from. For polypropylenetooling the temperature should be less than the melting point of theplastic. In one embodiment, for a water dispersion of between about 40to 50 percent solids and a polypropylene mold, the drying temperaturescan be between about 90° C. to about 165° C., or between about 105° C.to about 150° C., or between about 105° C. to about 120° C. Highertemperatures can lead to improved production speeds but can also lead todegradation of the polypropylene tooling limiting its useful life as amold.

The fifth process step involves removing resultant precursor shapedabrasive particles with from the mold cavities. The precursor shapedabrasive particles can be removed from the cavities by using thefollowing processes alone or in combination on the mold: gravity,vibration, ultrasonic vibration, vacuum, or pressurized air to removethe particles from the mold cavities.

The precursor abrasive particles can be further dried outside of themold. If the alpha alumina precursor dispersion is dried to the desiredlevel in the mold, this additional drying step is not necessary.However, in some instances it may be economical to employ thisadditional drying step to minimize the time that the alpha aluminaprecursor dispersion resides in the mold. Typically, the precursorshaped abrasive particles will be dried from 10 to 480 minutes, or from120 to 400 minutes, at a temperature from 50° C. to 160° C., or at 120°C. to 150° C.

The sixth process step involves calcining the precursor shaped abrasiveparticles. During calcining, essentially all the volatile material isremoved, and the various components that were present in the alphaalumina precursor dispersion are transformed into metal oxides. Theprecursor shaped abrasive particles are generally heated to atemperature from 400° C. to 800° C., and maintained within thistemperature range until the free water and over 90 percent by weight ofany bound volatile material are removed. In an optional step, it may bedesired to introduce the modifying additive by an impregnation process.A water-soluble salt can be introduced by impregnation into the pores ofthe calcined, precursor shaped abrasive particles. Then the precursorshaped abrasive particles are pre-fired again. This option is furtherdescribed in U.S. Pat. No. 5,164,348 (Wood).

The seventh process step involves sintering the calcined, precursorshaped abrasive particles to form alpha alumina particles. Prior tosintering, the calcined, precursor shaped abrasive particles are notcompletely densified and thus lack the desired hardness to be used asshaped abrasive particles. Sintering takes place by heating thecalcined, precursor shaped abrasive particles to a temperature of from1,000° C. to 1,650° C. and maintaining them within this temperaturerange until substantially all of the alpha alumina monohydrate (orequivalent) is converted to alpha alumina and the porosity is reduced toless than 15 percent by volume. The length of time to which thecalcined, precursor shaped abrasive particles must be exposed to thesintering temperature to achieve this level of conversion depends uponvarious factors but usually from five seconds to 48 hours is typical.

In another embodiment, the duration for the sintering step ranges fromone minute to 90 minutes. After sintering, the shaped abrasive particlescan have a Vickers hardness of 10 GPa, 16 GPa, 18 GPa, 20 GPa, orgreater.

Other steps can be used to modify the described process such as, forexample, rapidly heating the material from the calcining temperature tothe sintering temperature, centrifuging the alpha alumina precursordispersion to remove sludge and/or waste. Moreover, the process can bemodified by combining two or more of the process steps if desired.Conventional process steps that can be used to modify the process ofthis disclosure are more fully described in U.S. Pat. No. 4,314,827(Leitheiser).

Shaped abrasive particles used in the present disclosure may compriseplates, rods, or a combination thereof, for example. In preferredembodiments, the shaped abrasive particles have shapes that can becharacterized as thin bodies having triangular, rectangular (includingsquare), or other geometric shapes with sharp points. Such shapedabrasive particles have a front face and a back face, both of whichfaces have substantially the same geometric shape. The faces areseparated by the thickness of the particle. The ratio of the length ofthe shortest facial dimension of an abrasive particle to its thicknessis at least 1 to 1, preferably at least 2 to 1, more preferably at least5 to 1, and most preferably at least 6 to 1.

Preferred shaped abrasive particles are shaped as rectangular (includingsquare), or triangular plates, preferably having a sloping sidewall; forexample, triangular particles having a sloping sidewall as described inU.S. Pat. No. 8,142,531 (Adefris et al.). FIG. 3 shows an exemplary suchshaped abrasive particle 300 having the shape of a truncated triangularpyramid.

Further details concerning methods for making shaped abrasive particlesare described in U.S. U.S. Pat. No. 8,764,865 (Adefris et al.), U.S.Pat. No. 8,142,532 (Adefris et al.), U.S. Pat. No. 8,123,828 (Adefris etal.), U.S. Pat. No. 8,142,891 (Culler et al.), U.S. Pat. No. 5,366,523(Rowenhorst et al.), and U.S. Pat. No. 5,204,916 (Berg et al.), and inU.S. Publ. Patent Appln. No. 2009/0165394 A1 (Culler et al.) and2013/0040537 A1 (Erickson et al.).

The shaped abrasive particles used in the present disclosure cantypically be made using tools (i.e., molds) cut using diamond tooling,which provides higher feature definition than other fabricationalternatives such as, for example, stamping or punching Typically, thecavities in the tool surface have planar faces that meet along sharpedges, and form the sides and top of a truncated pyramid. The resultantshaped abrasive particles have a respective nominal average shape thatcorresponds to the shape of cavities (e.g., truncated pyramid) in thetool surface; however, variations (e.g., random variations) from thenominal average shape may occur during manufacture, and shaped abrasiveparticles exhibiting such variations are included within the definitionof shaped abrasive particles as used herein.

The shaped abrasive particles are typically selected to have a length ina range of from 0.001 mm to 26 mm, more typically 0.1 mm to 10 mm, andmore typically 0.5 mm to 5 mm, although other lengths may also be used.In some embodiments, the length may be expressed as a fraction of thethickness of the depressed center grinding wheel in which it iscontained. For example, the shaped abrasive particle may have a lengthgreater than half the thickness of the depressed center grinding wheel.In some embodiments, the length may be greater than the thickness of thedepressed center grinding wheel.

The shaped abrasive particles are typically selected to have a width ina range of from 0.001 mm to 26 mm, more typically 0.1 mm to 10 mm, andmore typically 0.5 mm to 5 mm, although other lengths may also be used.

The shaped abrasive particles are typically selected to have a thicknessin a range of from 0.005 mm to 1.6 mm, more typically, from 0.2 to 1.2mm.

In some embodiments, the shaped abrasive particles may have an aspectratio (length to thickness) of at least 2, 3, 4, 5, 6, or more.

The first abrasive particles (i.e., in the working layer) may containsolely of first shaped abrasive particles, or first shaped abrasiveparticles in combination with an amount of third crushed abrasiveparticles. Likewise, the second abrasive particles (i.e., in theintermediate layer) may contain solely of second shaped abrasiveparticles, or second shaped abrasive particles in combination with anamount of second crushed abrasive particles. In any event, the ratio ofthe weight percent of the first shaped abrasive particles in the firstabrasive particles to the weight percent of the second shaped abrasiveparticles in the second abrasive particles is from 40:60 to 60:40,preferably from 45:55 to 55:45.

The first and/or second abrasive particles may comprise more than onesize or shape of shaped abrasive particles, although a single size andshape is typically preferred. The first and second abrasive particlesmay be the same or different, preferably the same, with regard to shape,size, and/or composition.

Surface coatings on the shaped abrasive particles may be used to improvethe adhesion between the shaped abrasive particles and a binder materialin abrasive articles, or can be used to aid in electrostatic depositionof the shaped abrasive particles. In one embodiment, surface coatings asdescribed in U.S. Pat. No. 5,352,254 (Celikkaya) in an amount of 0.1 to2 percent surface coating to shaped abrasive particle weight may beused. Such surface coatings are described in U.S. Pat. No. 5,213,591(Celikkaya et al.); U.S. Pat. No. 5,011,508 (Wald et al.); U.S. Pat. No.1,910,444 (Nicholson); U.S. Pat. No. 3,041,156 (Rowse et al.); U.S. Pat.No. 5,009,675 (Kunz et al.); U.S. Pat. No. 5,085,671 (Martin et al.);U.S. Pat. No. 4,997,461 (Markhoff-Matheny et al.); and U.S. Pat. No.5,042,991 (Kunz et al.). Additionally, the surface coating may preventthe shaped abrasive particle from capping. Capping is the term todescribe the phenomenon where metal particles from the workpiece beingabraded become welded to the tops of the shaped abrasive particles.Surface coatings to perform the above functions are known to those ofskill in the art.

Useful crushed abrasive particles include, for example, crushedparticles of fused aluminum oxide, heat treated aluminum oxide, whitefused aluminum oxide, ceramic aluminum oxide materials such as thosecommercially available under the trade designation 3M CERAMIC ABRASIVEGRAIN from 3M Company of St. Paul, Minn., black silicon carbide, greensilicon carbide, titanium diboride, boron carbide, tungsten carbide,titanium carbide, diamond, cubic boron nitride, garnet, fused aluminazirconia, sol-gel derived abrasive particles, iron oxide, chromia,ceria, zirconia, titania, silicates, tin oxide, silica (such as quartz,glass beads, glass bubbles and glass fibers) silicates (such as talc,clays (e.g., montmorillonite), feldspar, mica, calcium silicate, calciummetasilicate, sodium aluminosilicate, sodium silicate), flint, andemery. Examples of sol-gel derived abrasive particles can be found inU.S. Pat. No. 4,314,827 (Leitheiser et al.), U.S. Pat. No. 4,623,364(Cottringer et al.); U.S. Pat. No. 4,744,802 (Schwabel), U.S. Pat. No.4,770,671 (Monroe et al.); and U.S. Pat. No. 4,881,951 (Monroe et al.).It is also contemplated that the abrasive particles could compriseabrasive agglomerates such, for example, as those described in U.S. Pat.No. 4,652,275 (Bloecher et al.) or U.S. Pat. No. 4,799,939 (Bloecher etal.).

Typically, conventional crushed abrasive particles are independentlysized according to an abrasives industry recognized specified nominalgrade. Exemplary abrasive industry recognized grading standards includethose promulgated by ANSI (American National Standards Institute), FEPA(Federation of European Producers of Abrasives), and JIS (JapaneseIndustrial Standard). Such industry accepted grading standards include,for example: ANSI 4, ANSI 6, ANSI 8, ANSI 16, ANSI 24, ANSI 30, ANSI 36,ANSI 40, ANSI 50, ANSI 60, ANSI 80, ANSI 100, ANSI 120, ANSI 150, ANSI180, ANSI 220, ANSI 240, ANSI 280, ANSI 320, ANSI 360, ANSI 400, andANSI 600; FEPA P8, FEPA P12, FEPA P16, FEPA P24, FEPA P30, FEPA P36,FEPA P40, FEPA P50, FEPA P60, FEPA P80, FEPA P100, FEPA P120, FEPA P150,FEPA P180, FEPA P220, FEPA P320, FEPA P400, FEPA P500, FEPA P600, FEPAP800, FEPA P1000, FEPA P1200; FEPA F8, FEPA F12, FEPA F16, and FEPA F24;and JIS 8, JIS 12, JIS 16, JIS 24, JIS 36, JIS 46, JIS 54, JIS 60, JIS80, JIS 100, JIS 150, JIS 180, JIS 220, JIS 240, JIS 280, JIS 320, JIS360, JIS 400, JIS 400, JIS 600, JIS 800, JIS 1000, JIS 1500, JIS 2500,JIS 4000, JIS 6000, JIS 8000, and JIS 10,000. More typically, thecrushed aluminum oxide particles and the non-seeded sol-gel derivedalumina-based abrasive particles are independently sized to ANSI 60 and80, or FEPA F36, F46, F54 and F60 or FEPA P60 and P80 grading standards.

Alternatively, shaped abrasive particles can be graded to a nominalscreened grade using U.S.A. Standard Test Sieves conforming to ASTM E-11“Standard Specification for Wire Cloth and Sieves for Testing Purposes”.ASTM E-11 prescribes the requirements for the design and construction oftesting sieves using a medium of woven wire cloth mounted in a frame forthe classification of materials according to a designated particle size.A typical designation may be represented as −18+20 meaning that theshaped abrasive particles pass through a test sieve meeting ASTM E-11specifications for the number 18 sieve and are retained on a test sievemeeting ASTM E-11 specifications for the number 20 sieve. In oneembodiment, the shaped abrasive particles have a particle size such thatmost of the particles pass through an 18 mesh test sieve and can beretained on a 20, 25, 30, 35, 40, 45, or 50 mesh test sieve. In variousembodiments, the shaped abrasive particles can have a nominal screenedgrade comprising: −18+20, −20/+25, −25+30, −30+35, −35+40, −40+45,−45+50, −50+60, −60+70, −70/+80, −80+100, −100+120, −120+140, −140+170,−170+200, −200+230, −230+270, −270+325, −325+400, −400+450, −450+500, or−500+635. Alternatively, a custom mesh size could be used such as−90+100.

In some embodiments, some or all of the abrasive particles (shapedand/or crushed) are treated with a coupling agent (e.g., an organosilanecoupling agent) to enhance adhesion of the abrasive particles to thebinder. Coupling agents are well-known to those of skill in the abrasivearts. Examples of coupling agents include trialkoxysilanes (e.g.,gamma-aminopropyltriethoxysilane), titanates, and zirconates. Theabrasive particles may be treated before combining them with the bindermaterial, or they may be surface treated in situ by including a couplingagent to the binder material.

In some embodiments, depressed center grinding wheels according to thepresent disclosure contain additional grinding aids such as, forexample, polytetrafluoroethylene particles, cryolite, sodium chloride,FeS₂ (iron disulfide), or KBF₄; typically in amounts of from 1 to 25percent by weight, more typically 10 to 20 percent by weight, subject toweight range requirements of the other constituents being met. Grindingaids are added to improve the cutting characteristics of the cut-offwheel, generally by reducing the temperature of the cutting interface.The grinding aid may be in the form of single particles or anagglomerate of grinding aid particles. Examples of precisely shapedgrinding aid particles are taught in U.S. Patent Publ. No. 2002/0026752A1 (Culler et al.).

In some embodiments, the organic binder materials may containplasticizer such as, for example, that available as SANTICIZER 154PLASTICIZER from UNIVAR USA, Inc. of Chicago, Ill.

Depressed center grinding wheels according to the present disclosure maycontain additional components such as, for example, filler particles,subject to weight range requirements of the other constituents beingmet. Filler particles may be added to occupy space and/or provideporosity. Porosity enables the depressed center grinding wheel to shedused or worn abrasive particles to expose new or fresh abrasiveparticles.

Depressed center grinding wheels according to the present disclosurehave any range of porosity; for example, from about 1 percent to 50percent, typically 1 percent to 40 percent by volume. Examples offillers include bubbles and beads (e.g., glass, ceramic (alumina), clay,polymeric, metal), cork, gypsum, marble, limestone, flint, silica,aluminum silicate, and combinations thereof.

Depressed center grinding wheels according to the present disclosure areuseful, for example, as Type 27 (e.g., as in American National StandardsInstitute standard ANSI B7.1-2000 (2000) in section 1.4.14)depressed-center grinding wheels.

Depressed center grinding wheels according to the present disclosure aretypically 0.80 millimeter (mm) to 16 mm in thickness, more typically 1mm to 8 mm, and typically have a diameter between 2.5 cm and 100 cm (40inches), more typically between about 7 cm and 13 cm, although otherdimensions may also be used (e.g., wheels as large as 100 cm in diameterare known). An optional center hole may be used to attaching thedepressed center grinding wheel to a power driven tool. If present, thecenter hole is typically 0.5 cm to 2.5 cm in diameter, although othersizes may be used. The optional center hole may be reinforced; forexample, by a metal flange. Alternatively, a mechanical fastener may beaxially secured to one surface of the cut-off wheel. Examples includethreaded posts, threaded nuts, Tinnerman nuts, and bayonet mount posts.

As discussed previously, depressed center grinding wheels according tothe present disclosure include at least two scrims that reinforce thedepressed center grinding wheel. Examples of scrims include woven orknitted cloth, mesh, and screens. The scrim may comprise glass fibers(e.g., fiberglass), organic fibers such as polyamide, polyester, orpolyimide. The scrim may comprise an open mesh selected from the groupconsisting of woven, nonwoven, or knitted fiber mesh; synthetic fibermesh; natural fiber mesh; metal fiber mesh; molded thermoplastic polymermesh; molded thermoset polymer mesh; perforated sheet materials; slitand stretched sheet materials; and combinations thereof. The scrim neednot be woven in a uniform pattern but may also include a nonwoven randompattern. Thus, the openings may either be in a pattern or randomlyspaced. The scrim network openings may be rectangular or they may haveother shapes including a diamond shape, a triangular shape, an octagonalshape or a combination of shapes.

In some instances, it may be desirable to include reinforcing staplefibers within the bonding medium, so that the fibers are homogeneouslydispersed throughout the grinding wheel.

Depressed center grinding wheels according to the present disclosure areuseful, for example, for abrading a workpiece. During use, the depressedcenter grinding wheel can be used dry or wet. During wet grinding, thewheel is used in conjunction with water, oil-based lubricants, orwater-based lubricants. Depressed center grinding wheels according tothe present disclosure may be particularly useful on various workpiecematerials such as, for example, carbon steel sheet or bar stock and moreexotic metals (e.g., stainless steel or titanium), or on softer moreferrous metals (e.g., mild steel, low alloy steels, or cast irons).

Depressed center grinding wheels according to the present disclosure areuseful for grinding a workpiece at an acute angle with the workpiece.Such an abrading process is shown in FIG. 4, wherein depressed centergrinding wheel 100 abrades workpiece 400. During grinding, the working,intermediate, and back layers experience wear and participate in theabrading of the workpiece.

SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE

In a first embodiment, the present disclosure provides a depressedcenter grinding wheel comprising an abrasive disc having a workingsurface and a back surface opposite the working surface, wherein theworking surface has a depressed center portion, and wherein the abrasivedisc comprises: a working layer comprising first abrasive particlesretained in a first binder material, the first abrasive particlescomprising first shaped abrasive particles, wherein the first shapedabrasive particles comprise from 40 to 100 weight percent of the firstabrasive particles;

a back layer comprising second abrasive particles retained in a secondbinder material comprising first crushed abrasive particles andessentially free of shaped abrasive particles;

an intermediate layer disposed between the working layer and the backlayer, the intermediate layer comprising third abrasive particlesretained in a third binder material, the intermediate layer comprisingsecond shaped abrasive particles and second crushed abrasive particles,wherein the second shaped abrasive particles comprise 25 to 75 weightpercent of the second abrasive particles;

a first reinforcing scrim sandwiched between the back layer and theintermediate layer; and

a second reinforcing scrim adjacent one of:

-   -   the back layer opposite the intermediate layer;    -   the intermediate layer opposite the back layer; or    -   the working layer opposite the intermediate layer.

In a second embodiment, the present disclosure provides a depressedcenter grinding wheel according to the first embodiment, wherein thesecond reinforcing scrim is sandwiched between the working layer and theintermediate layer.

In a third embodiment, the present disclosure provides a depressedcenter grinding wheel according to the second embodiment, furthercomprising a third reinforcing scrim bonded to the working layeropposite the intermediate layer.

In a fourth embodiment, the present disclosure provides a depressedcenter grinding wheel according to the first embodiment, wherein thesecond reinforcing scrim is secured to the back layer opposite theintermediate layer.

In a fifth embodiment, the present disclosure provides a depressedcenter grinding wheel according to the fourth embodiment, furthercomprising a third reinforcing scrim sandwiched between the intermediatelayer and the working layer.

In a sixth embodiment, the present disclosure provides a depressedcenter grinding wheel according to the fourth or fifth embodiment,further comprising a fourth reinforcing scrim bonded to the workinglayer opposite the intermediate layer.

In a seventh embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to sixthembodiments, wherein the first shaped abrasive particles comprisetriangular shaped abrasive particles.

In an eighth embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to seventhembodiments, wherein the second shaped abrasive particles comprisetriangular shaped abrasive particles.

In a ninth embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to eighthembodiments, wherein the ratio of the weight percent of the first shapedabrasive particles in the first abrasive particles to the weight percentof the second shaped abrasive particles in the second abrasive particlesis from 40:60 to 60:40.

In a tenth embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to eighthembodiments, wherein the ratio of the weight percent of the first shapedabrasive particles in the first abrasive particles to the weight percentof the second shaped abrasive particles in the second abrasive particlesis from 45:55 to 55:45.

In an eleventh embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to tenthembodiments, wherein the first and second shaped abrasive particlescomprise alpha alumina.

In a twelfth embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to eleventhembodiments, further comprising a centrally disposed arbor holeextending through the abrasive disc.

In a thirteenth embodiment, the present disclosure provides a depressedcenter grinding wheel according to any one of the first to twelfthembodiments, further comprising an attachment member centrally disposedon the back surface of the abrasive disc.

Objects and advantages of this disclosure are further illustrated by thefollowing non-limiting examples, but the particular materials andamounts thereof recited in these examples, as well as other conditionsand details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in theExamples and the rest of the specification are by weight.

The following abbreviations are used for materials in the examples.

TABLE OF ABBREVIATIONS ABBREVIATION DESCRIPTION AP1 grade 36 aluminumoxide abrasive particles, obtained under trade designation “36 BFRPL”from Treibacher Schleifmittel AG, Villach, Austria. AP2 a grade 36+precision-shaped ceramic alumina abrasive particle prepared according tothe procedure described hereinbelow. AP3 grade 24 aluminum oxideabrasive particles, obtained under trade designation “24 BFRPL” fromTreibacher Schleifmittel AG, Villach, Austria. PR1 liquid phenolicresin, obtained under trade designation “PREFERE 825136G1” from Dynea OyCorporation, Helsinki, Finland. PR2 phenolic resin powder (a solidphenolic resin), obtained under trade designation “VARCUM 29302” fromDurez Corporation, Dallas, Texas. CRY Sodium hexafluoroaluminate,obtained under trade designation “CRYOLITE” from Freebee, Ullerslev,Denmark. SCRIM1 fiberglass mesh having the trade designation “STYLE4400” from Industrial Polymer and Chemicals, Inc., Shrewsbury,Massachusetts. SCRIM2 fiberglass mesh having the trade designation“STYLE 184” from Industrial Polymer and Chemicals, Inc.. HC5 0.7 micronaluminum trihydroxide particles available as HYDRAL COAT 7 from Almatis,Inc., Leetsdale, Pennsylvania.

Preparation of AP2

Shaped abrasive particles were prepared according to the disclosure ofU.S. Pat. No. 8,142,531 (Adefris et al.). The shaped abrasive particleswere prepared by molding alumina sol gel in equilateral triangle-shapedpolypropylene mold cavities of 0.028 inch (0.71 millimeter) depth and0.11 inch (0.28 millimeter) on each side. The draft angle α between thesidewall and bottom of the mold was 98 degrees. After drying and firing,the shaped particles were calcined at approximately 650° C., and thensaturated with a magnesium nitrate solution (10.5 percent by weight asmagnesium oxide, and having 0.02 percent by weight of HC5 dispersedtherein). Excess nitrate solution was removed, and the saturated shapedparticles were allowed to dry after which the particles were againcalcined at 650° C. and sintered at approximately 1400° C. resulting inshaped ceramic abrasive particles. Both the calcining and sintering wereaccomplished using rotary tube kilns.

Grinding Test

Abrasive wheels were tested by grinding a rectangular mild steel bar(0.25 inch (0.6 cm)×18 inches (45.7 cm)×3 inches (7.6 cm)) over a 0.25inch (0.6 cm)×18 inches (45.7 cm) area of the surface while mounted on a12000 rpm air driven grinder that oscillated back and forth (onecycle=18 inches (45.7 cm) each way for a total of 36 inches (91 cm)) forten one-minute cycles. The applied load was the grinder weight of 9pounds (4.1 kg) and the abrasive wheel was held at an angle of 15degrees relative to the surface (i.e., 0 degrees). The steel bar wasweighed before and after each cycle, and the weight loss (i.e., cut) wasrecorded. The steel bar was traversed 16 times from end to end percycle. Weight loss from the grinding disc (i.e., disc wear) was recordedafter each 10-cycle test.

Example 1 and Comparative Examples a-b

Mixes were prepared according to the amounts and components listed inTable 1. Mix 1, Mix 2 and Mix 4 were prepared by combining the indicatedcomponents using a paddle-type mixer (obtained as “CUISINART SM-70” fromConair Corporation, East Windsor, N.J., operated at speed 1) for 10minutes. Mix 3 was prepared by combining Mix 1 and Mix 2 using apaddle-type mixer for 10 minutes. Mix 5 was prepared by combining Mix 4and Mix 2 using a paddle-type mixer for 10 minutes. Mix 6 was preparedby combining 50% Mix 1 and 50% Mix 4 using a paddle-type mixer for 10minutes.

TABLE 1 AMOUNT IN GRAMS COMPONENT Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6AP1 1000 — 1000  — — 500 AP2 — — — 1000 1000  500 PR1  105 — 105  105105 105 PR2 — 194 194 — 194 — CRY — 200 200 — 200 —

Example 1

A Type 27 depressed-center composite grinding wheel was prepared asfollows. A 4.5-inch (11.4 centimeters) diameter disc of SCRIM′ wasplaced into a 4.5-inch (11.4 centimeters) diameter cavity die. Mix 3 (50grams) was spread out evenly. A second 4-inch (10.2 centimeters)diameter of SCRIM2 was placed on top of Mix 3. Mix 6 (50 grams) of wasspread out evenly and a third 4-inch (10.2 centimeters) diameter ofSCRIM2 was placed on top of Mix 6. Then Mix 5 (50 grams) of was spreadout evenly. The filled cavity mold was then pressed at a pressure of 40tons/38 square inches (14.5 megapascals).

The resulting wheel was removed from the cavity mold and placed on aspindle between depressed center aluminum plates in order to be pressedinto a Type 27 depressed-center grinding wheel. The wheel was compressedat 5 ton/38 square inches (1.8 megapascals) to shape the disc. The wheelwas then placed in an oven to cure for 7 hours at 79° C., 3 hours at107° C., 18 hours at 185° C., and a temperature ramp-down over 4 hoursto 27° C. The dimensions of the final grinding wheel were 180 millimeterdiameter×7 millimeter thickness. The center hole was ⅞ inch (2.2centimeters) in diameter. The resultant depressed-center compositegrinding wheel was configured such that a layer of Mix 5 was the workinglayer.

Comparative Example A

Comparative Example A was a Type 27 depressed-center grinding wheelprepared according to the procedure of Example 1, except that Mix 5 wasused instead of Mix 6 in middle layer (so that Mix 5 was used in bothmiddle and top layers). The resultant depressed-center compositegrinding wheel was configured such that a layer of Mix 5 was the workinglayer.

Comparative Example B

Comparative Example B was a Type 27 depressed-center grinding wheelprepared according to the procedure of Example 1, except that Mix 6 wasused instead of Mix 5 in top layer (so that Mix 6 was used in bothmiddle and top layers). The resultant depressed-center compositegrinding wheel was configured such that a layer of Mix 6 was the workinglayer.

Grinding test results of Example 1 and Comparative Examples A and B arereported in Table 2 (below).

TABLE 2 COMPARATIVE COMPARATIVE EXAMPLE 1 EXAMPLE A EXAMPLE B Total Cut423.06 422.72 364.05 After 10 Cycles (grams) Disc Wear 5.5 5.5 7 After10 Cycles (grams)

Examples 2-7 and Comparative Examples C-E

Mixes were prepared according to the amounts and components listed inTable 3. Mix 7, Mix 8 and Mix 10 were prepared by combining theindicated components using a paddle-type mixer (CUISINART SM-70 fromConair Corporation, East Windsor, N.J., operated at speed 1) for 10minutes. Mix 9 was prepared by combining Mix 7 and Mix 8 using apaddle-type mixer for 10 minutes. Mix 11 was prepared by combining Mix10 and Mix 8 using a paddle-type mixer for 10 minutes. Mix 12 wasprepared by combining 25% Mix 9 and 75% Mix 11 using a paddle-type mixerfor 10 minutes. Mix 13 was prepared by combining 50% Mix 9 and 50% Mix11 using a paddle-type mixer for 10 minutes. Mix 14 was prepared bycombining 75% Mix 9 and 25% Mix 11 using a paddle-type mixer for 10minutes.

TABLE 3 AMOUNT IN GRAMS COM- Mix Mix Mix Mix Mix Mix Mix PONENT 7 8 Mix9 10 11 12 13 14 AP3 1290 — 1290  — — 322 645 968 AP2 — — — 1290 1290 968 645 322 PR1  83 —  83  83  83 83 83 83 PR2 — 233 233 — 233 233 233233 CRY — 233 233 — 233 233 233 233

A Type 27 depressed-center composite grinding wheel was prepared foreach Example in Examples 2-7 and Comparative Examples C-E as follows.Mixes used in each Example as bottom, middle and top layers and theiramounts are reported in Table 4. A 4.5-inch (11.4-cm) diameter disc ofSCRIM′ was placed into a 4.5-inch (11.4-cm) diameter cavity die. Bottomlayer mix was spread out evenly. A second 4.0-inch (10.2-cm) diameter ofSCRIM2 was placed on top of bottom layer mix. The middle layer mix wasspread out evenly and then top layer mix was spread out evenly. A third3-inch (7.6-cm) diameter of SCRIM2 was placed on top of top layer mix.The filled cavity mold was then pressed at a pressure of 40 tons/38square inches (14.5 mPa).

The resulting wheel was removed from the cavity mold and placed on aspindle between depressed center aluminum plates in order to be pressedinto a Type 27 depressed-center grinding wheel. The wheel was compressedat 5 ton/38 square inches (1.8 mPa) to shape the disc. The wheel wasthen placed in an oven to cure for 7 hours at 79° C., 3 hours at 107°C., 18 hours at 185° C., and a temperature ramp-down over 4 hours to 27°C. The dimensions of the final grinding wheel were 180 millimeterdiameter×7 millimeter thickness. The center hole was ⅞ inch (2.2 cm) indiameter. The resultant depressed-center composite grinding wheel wasconfigured such that a top layer was the working layer.

Grinding test results of Example 2-7 and Comparative Examples C-E arereported in Table 4 (below).

TABLE 4 Comparative Comparative Comparative Example 2 Example 3 ExampleC Example 4 Example 5 Example D Example 6 Example 7 Example E Bottom Mix9, Mix 9, Mix 9, Mix 9, Mix 9, Mix 9, Mix 9, Mix 9, Mix 9, Layer Mix 50grams 50 grams 50 grams 50 grams 50 grams 50 grams 50 grams 50 grams 50grams Middle Mix 13, Mix 13, Mix 12, Mix 13, Mix 13, Mix 13, Mix 14, Mix14, Mix 14, Layer Mix 50 grams 70 grams 50 grams 50 grams 70 grams 50grams 50 grams 70 grams 50 grams Top Layer Mix 11, Mix 11, Mix 12, Mix12, Mix 12, Mix 13, Mix 13, Mix 13, Mix 14, Mix 50 grams 30 grams 50grams 50 grams 30 grams 50 grams 50 grams 30 grams 50 grams GrindingTest Results Total Cut 350.90 317.58 287.57 319.31 323.40 242.84 275.19266.81 200.36 After 10 Cycles, grams Disc Wear 3.67 3.11 3.45 3.36 3.583.91 3.34 3.16 2.59 After 10 Cycles, grams

Various modifications and alterations of this disclosure may be made bythose skilled in the art without departing from the scope and spirit ofthis disclosure, and it should be understood that this disclosure is notto be unduly limited to the illustrative embodiments set forth herein.

1-13. (canceled)
 14. A depressed center grinding wheel comprising anabrasive disc having a working surface and a back surface opposite theworking surface, wherein the working surface has a depressed centerportion, and wherein the abrasive disc comprises: a working layercomprising first abrasive particles retained in a first binder material,the first abrasive particles comprising first shaped abrasive particles,wherein the first shaped abrasive particles comprise from 40 to 100weight percent of the first abrasive particles; a back layer comprisingsecond abrasive particles retained in a second binder materialcomprising first crushed abrasive particles and essentially free ofshaped abrasive particles; an intermediate layer disposed between theworking layer and the back layer, the intermediate layer comprisingthird abrasive particles retained in a third binder material, theintermediate layer comprising second shaped abrasive particles andsecond crushed abrasive particles, wherein the second shaped abrasiveparticles comprise 25 to 75 weight percent of the second abrasiveparticles; a first reinforcing scrim sandwiched between the back layerand the intermediate layer; and a second reinforcing scrim adjacent oneof: the back layer opposite the intermediate layer; the intermediatelayer opposite the back layer; or the working layer opposite theintermediate layer.
 15. The depressed center grinding wheel of claim 14,wherein the second reinforcing scrim is sandwiched between the workinglayer and the intermediate layer.
 16. The depressed center grindingwheel of claim 15, further comprising a third reinforcing scrim bondedto the working layer opposite the intermediate layer.
 17. The depressedcenter grinding wheel of claim 14, wherein the second reinforcing scrimis secured to the back layer opposite the intermediate layer.
 18. Thedepressed center grinding wheel of claim 17, further comprising a thirdreinforcing scrim sandwiched between the intermediate layer and theworking layer.
 19. The depressed center grinding wheel of claim 17,further comprising a fourth reinforcing scrim bonded to the workinglayer opposite the intermediate layer.
 20. The depressed center grindingwheel of claim 14, wherein the first shaped abrasive particles comprisetriangular shaped abrasive particles.
 21. The depressed center grindingwheel of claim 14, wherein the second shaped abrasive particles comprisetriangular shaped abrasive particles.
 22. The depressed center grindingwheel of claim 14, wherein the ratio of the weight percent of the firstshaped abrasive particles in the first abrasive particles to the weightpercent of the second shaped abrasive particles in the second abrasiveparticles is from 40:60 to 60:40.
 23. The depressed center grindingwheel of claim 14, wherein the ratio of the weight percent of the firstshaped abrasive particles in the first abrasive particles to the weightpercent of the second shaped abrasive particles in the second abrasiveparticles is from 45:55 to 55:45.
 24. The depressed center grindingwheel of claim 14, wherein the first and second shaped abrasiveparticles comprise alpha alumina.
 25. The depressed center grindingwheel of claim 14, further comprising a centrally disposed arbor holeextending through the abrasive disc.
 26. The depressed center grindingwheel of claim 14, further comprising an attachment member centrallydisposed on the back surface of the abrasive disc.